1. Field of the Invention
The present invention relates to an image sensing apparatus having an image scanner for sensing image information of a document, and more particularly, to an image sensing apparatus capable of recording an image precisely according to the image information obtained by scanning a document with an image scanner and also to a method of making a shading correction.
2. Description of the Related Art
Thermal transfer printers are now widely used. A typical thermal transfer printer operates as follows. That is, a recording medium is pinched between a carrying roller and a pressing roller which pressing the recording medium against the carrying roller, thereby carrying the recording medium to a location between a platen and a thermal head serving as recording means. Then, the thermal head mounted on a carriage is pressed against the recording medium at a predetermined amount of pressure. While maintaining the above pressure, the carriage is moved and the thermal head is activated in accordance with a given recording signal wherein an ink ribbon disposed in a ribbon cassette mounted on the carriage is wound during the above operation. As a result, the ink on the ink ribbon is melted and transferred to the recording medium and thus a desired image is recorded on the recording medium.
An image sensing means called an image scanner is used in a peripheral device of a computer system or used in a recording apparatus. In the image scanner, a document is illuminated by light emitted by a light source, and the light reflected from the document is sensed by a sensor device such as a CCD having a plurality of sensor elements arranged in an array form. Thus, the image information of the document is converted into an electric signal and the result is output.
The inventors of the present invention have developed a thermal transfer printer having an image scanner mounted on a carriage, wherein a document is carried between a platen and the carriage which is moved across the document thereby scanning the document by the image scanner. Thus, the image information of the document is sensed by the image scanner, and an image is recorded on a recording medium in accordance with the image information sensed.
The sensing of the image information of the document via the image scanner is performed as follows. The document is illuminated by light emitted from a light source member disposed on the image scanner. The image of the document is then sensed by detecting the light reflected from the document. Since the light source member includes a plurality of lamps or the like, the light rays emitted from those lamps partially overlap each other on the document. The overlapped areas produce a greater intensity of reflected light than the areas which are exposed to a single light ray without having an overlap. As a result, the image information sensed by the image scanner includes brightness information different from the true brightness of the image. That is, the sensed image becomes brighter in the overlapped areas than the true brightness, while the brightness of the sensed image for the other areas becomes lower than the brightness of the overlapped areas.
A similar variation in the output of the scanner can also occur due to other factors such as a variation in the intensity of light rays from the light source, an error of a lens, and a variation in the distance between the document and the CCD.
To avoid the problem of the variation in the output of the scanner, a correction is generally made as follows. That is, a white reference plate is prepared, and the brightness and the color (reference information) of the white reference plate is sensed before sensing the image information of a document via the image scanner. Correction reference data is then determined so that the output level of each CCD element for the reference information of the white reference plate becomes equal to a maximum level. Using this correction reference data, a shading correction is made so that the output level of each CCD element for the same color, which varies from element to element, is corrected to an equal level.
FIG. 8 illustrates the shading correction process. In FIGS. 8a-8d, each horizontal axis represents the location of CCD elements from one end to the opposite end of the sensor device of the image scanner. Each vertical axis represents the output level (gray level in the range from 0 to 255) of the respective CCD elements. When the white reference plate is sensed by the image scanner, those CCD elements located between two lamps of the image scanner have a high output level, while the CCD elements located outside either lamp have a low output level, as shown in FIG. 8a.
As can be seen from FIG. 8a, the output signal of the sensor of the image scanner has a curved characteristic resulting from the fact that CCD elements located in the central area have a high output level while those located near either end have a low output level.
To compensate for such the variation in the output level, a shading correction is made in such a manner that correction coefficients (having a value in the range of 0.5 to 1.5) are determined as shown in FIG. 8b such that the plots of correction coefficients are curved in an opposite direction to the characteristic curve of the sensor output signal shown in FIG. 8a, and the sensor output signal having such the characteristic shown in FIG. 8a is multiplied by the corresponding correction coefficient shown in FIG. 8b. As a result of the shading correction, the output of the sensor becomes flat along the X axis as shown in FIG. 8c.
In the conventional printer, however, when a document to be sensed by the image scanner is placed on a sensing position, if the document has an angle relative to the plane of the white reference plate, the output signal of the sensor obtained on the assumption that the white reference plate is located in a plane exactly identical to the document plane has an inclined linear characteristic which increases with the location toward the right as shown in FIG. 8d. This occurs even if the shading correction is made. Besides, the maximum-to-minimum difference in the level of the output signal in FIG. 8d becomes greater than that in FIG. 8a.
If the illumination area is illuminated by nonuniform intensity of light emitted from each lamp of the image scanner, when a photographic image is scanned by the image scanner, nonuniformity in gray level called white nonuniformity can occur in an image area having an achromatic color such as white or gray with high brightness.
As described above, if correction is performed simply according to the above shading correction technique, the image recorded in accordance with the image information obtained via the image scanner has a gray level error at a recording element location above or below that corresponding to the sensor element. This causes degradation in the quality of the recorded image.
Furthermore, in the conventional shading correction technique described above, when an image with an intermediate gray level between the white level with the highest brightness (highest light intensity) and the black level with the lowest brightness (lowest light intensity) is sensed, the output level of each CCD element has a deviation from its ideal value.
That is, although the conventional shading correction technique can eliminate the white level variation among CCD elements by making a correction so that each CCD element has an equal output level for the white level, and no error occurs in the black level since the black level equally has a brightness of zero, an error occurs in the output level of the CCD element when an image with an intermediate gray level between the lowest brightness (black) and the highest brightness (white) is sensed. As shown in FIG. 10, the output level of each CCD element does not vary in proportion to the brightness (intensity of incident light). That is, the relationship between the intensity of incident light and the output level is not linear. Furthermore, such the relationship varies from one CCD element to another. As a result, even when an image with an uniform gray level is sensed by CCD elements, the output level varies from one CCD element to another, and thus nonuniformity in gray level occurs. The above errors is caused by the variation in the characteristics such as sensitivity among CCD elements and also the variation in the intensity of reflected light depending on the location.